Many of the issues involved in sending communication signals over DC power lines are similar to sending such signals over the AC power grid. For example, the properties of the transmission medium can vary greatly. As another example, the amount and nature of noise sources are very difficult to predict.
However, there are some important differences too. While on the AC power grid, loads are mostly of an inductive nature, and some loads may be resistive and a few subtly capacitive. On a DC power bus, virtually all loads and sources are capacitive in nature. Another key difference lies in the use of transformers to convert between different voltage levels on an AC power grid that cannot be similarly employed on a DC bus. Hence, a majority of sources and loads on a DC bus employ electronic DC-DC converters, which are very strong noise emitters.
There are many challenges to communicating digital data quickly and reliably over a set of wires intended to conduct electrical energy. For a variety of reasons, the signal carrier frequency should not exceed 500 kHz. One reason is to avoid antenna effects, and avoid excessive signal attenuation and unintentional electromagnetic radiation. Power line carrier frequency is typically between 90 kHz and 490 kHz for a small power line network as might be used for a residence. Larger commercial power line installations must further limit the maximum carrier frequency so that the effective length of the wires does not exceed ⅛ of the carrier wavelength.
Low carrier frequency, in turn, means that the rate of signal that can be modulated on such a carrier is also very low to maintain a certain payload to carrier ratio. The signal rate that can be carried by a frequency is also limited by severe noise and attenuation typical of power lines because the slower the signal rate, the more energy is carried by a single symbol (bit). With more energy in a bit, the less likely it will be corrupted in transmission.
There are several types of products on the market at this time to address the needs of power line communications. For example, broadband communication products deliver local area network performance but often can not handle more than just a handful of nodes on the network. Broadband communication products also often lack the robustness and reliability that is needed for a sensor network.